Summary/Abstract Non-alcoholic fatty liver disease (NAFLD) represents a spectrum of excess fat accumulation in the liver (steatosis) without or with inflammation/fibrosis (non-alcoholic steatohepatitis - NASH). NAFLD is commonly observed in obesity and type 2 diabetes, but is also observed in non-obese patients associated with cardiovascular disease. To date, there are no proven medical therapies to reverse NAFLD. In order to develop effective treatments, it is critical to understand the basic mechanisms controlling hepatic fat accumulation and associated liver damage. Since both clinical and experimental studies demonstrate hepatic growth hormone (GH)-signaling is reduced in NAFLD and increasing GH can reduce NAFLD both in humans and mice, our recent work has focused on determining how GH mediates these effects. We have used unique mouse models with adult-onset loss of the hepatocyte GH receptor (GHR) and demonstrated GH signaling controls steatosis by suppressing de novo lipogenesis (DNL), an action of GH not previously appreciated. Additional preliminary data, coupled with published reports, led us to formulate the HYPOTHESIS: GH directly controls hepatocyte DNL via GHR/Stat5b-mediated regulation of glycolysis, to protect the liver from injury. The following Specific Aims (SA) will test this hypothesis. SA1 - Determine if the GHR signals through Stat5 to directly suppress hepatic DNL and prevent NAFLD progression. Expt SA1A, will determine if enhancing Stat5b activity can suppress DNL and prevent NAFLD by expressing a constitutively active form of Stat5b (Stat5bCA) in mice lacking the hepatocyte GHR. Since Stat5b enhances hepatic IGF1 expression and raises circulating IGF1 levels, Stat5bCA will be expressed in mice with combined knockdown of hepatocyte GHR and IGF1, to determine what actions of Stat5b are IGF1-independent. Expt SA1B, will establish if GHR-mediated Stat5b activation is required to control DNL, by expressing a mutant GHR in the hepatocyte of adult mice lacking the endogenous GHR, where this mutant receptor cannot activate Stat5b, but can active other JAK2-dependent and independent pathways. In a subset of mice, hepatocyte IGF1 expression will be reconstituted, to prevent secondary changes due to IGF1 loss. SA2 ? Determine if loss of hepatocyte GHR/Stat5b signaling acts independent of canonical insulin signaling to augment glycolysis-driven DNL. Expt SA2A, will determine if hepatocyte GHR loss and restoration of Stat5b activity regulates glycolytic flux and DNL when insulin and carbohydrate input to the liver is controlled, by assessing [1-13C] glucose enrichment of glycolytic, TCA intermediates and newly formed fatty acids, under hyperinsulinemic-hyperglycemic clamp conditions, in mice lacking the hepatocyte GHR, without or with Stat5bCA. Expt SA2B, will establish if the glycolysis-mediated transcription factor, Chrebp?, is required to enhance DNL, steatosis and liver injury after hepatocyte GHR loss, by generating mice with adult-onset, hepatocyte knockdown of both Chrebp? and GHR. Completion of these studies will enhance our basic understanding of the mechanisms by which GH directly controls hepatocyte carbohydrate/lipid metabolism and protects the liver against injury, with the ultimate goal of unveiling novel ?druggable? targets to treat NAFLD.